1. Field of the Invention
The present invention relates to the improvement of a ball screw, particularly a ball screw with a cage type retainer.
2. Background of the Related Art
In a usual ball screw having no retainer shown in FIG. 10, for example, balls 50 are disposed at a high density in a state that balls contact one another within the ball screw grooves 22 of a screw shaft 20, within the ball screw grooves 32 of a ball nut 30, and within the ball circulation path 62 of a circulation dowel 60. When the respective balls roll, the adjacent balls rotate to the opposite direction to each other at the contact portions thereof. Thus, since the adjacent balls slip at the contact portions thereof, the balls are prevented from rolling freely due to the so-called rolling conflict phenomenon, whereby the torque resistance becomes large and so there arises torque fluctuation. Such torque fluctuation due to the rolling conflict phenomenon among the balls generally deteriorates in a low speed operation and an oscillation or swinging operation. Accordingly, the torque fluctuation due to the rolling conflict phenomenon becomes a problem to be solved in particular when the ball screw is used in an electric discharge machine or a wire cutting machine which a low speed and oscillating operation is required, for example. Further, the torque fluctuation due to the rolling conflict phenomenon must be suppressed since is becomes a cause of disturbance of motor control in a machining center in which the contour controllability is considered to be important.
As the countermeasure of such torque fluctuation due to the rolling conflict phenomenon, it is required to prevent the direct contact among the balls to eliminate the rolling conflict phenomenon thereby to improve the torque efficiency. For example, the ball screw using such a ball retainer H shown in FIG. 11 has been known as the countermeasure of the torque fluctuation due to the rolling conflict phenomenon. Such a retainer H is formed by a cylindrical portion H1 which is provided with a multiplicity of round holes H2 for holding balls so as to be free to roll therein. The balls are disposed within the round holes H2 so as to be free to roll therein, and the retainer H thus configured is attached between a screw shaft and a ball nut. According to such a retainer H, since the balls do not contact one another, the aforesaid torque fluctuation due to the slip resistance among the balls does not occur. However, when the retainer H is used in such a state that the screw shaft rotates at the predetermined position but the ball nut does not rotate, the retainer moves together with the ball nut toward the axial direction in accordance with the rotation of the screw shaft (in this case, an amount of the movement of the retainer is a half of that of the ball nut). Thus, when the moving stroke of the ball nut is large, the retainer H may come out of the spiral groove of the ball nut and then balls may come out of the ball screw.
In view of the aforesaid problem, a ball screw with a cage type retainer eliminating the aforesaid problem has been proposed in Japanese Utility Model Examined Publication No. Hei. 2-5145, for example. This ball screw is provided with a cage type retainer 40 which is loosely fitted between a screw shaft and a ball nut and is arranged in a manner that a multiplicity of elongated holes 42 each extending toward the axial direction are formed at a cylindrical body portion thereof with a thin thickness as shown in FIG. 12. Balls are disposed within the elongated holes 42 so as to be free to roll therein. The ball nut is provided with a ball circulation path 62 of the inner circulation type which is formed like the ball nut shown in FIG. 10 in a manner that a hole portion extending over the adjacent two of ball screw grooves 32 is formed in the ball nut and a ball circulation dowel 60 is fitted into the hole portion. The balls move toward the axial direction within the respective elongated holes 42 of the cage type retainer 40 while rolling along the ball screw grooves. When the balls pass the ball circulation path 62 of the ball circulation dowel 60, the balls move over the thread (land portion) of the screw shaft, and so the balls repeat the inner circulation. Each of the elongated holes 42 of the cage type retainer 40 is set to have such a length that the balls are movable toward the axial direction relative to the retainer so as not to interfere the internal circulation of the balls. Such a ball screw with a cage type retainer is advantageous in that the amount of the torque fluctuation is small and the moving stroke of the ball nut is not limited by the rolling balls.
However, the aforesaid ball screw with a cage type retainer has the following problems to be solved.
(1) The dowel 60 serving as a circulation part is generally designed to have such a size that the inner diameter side of the dowel protrudes to the inner diameter side of the ball nut so as to smoothly pick up the balls. Thus, the cage type retainer 40 disposed in the space between the screw shaft and the ball nut may interfere with the inner diameter side of the dowel 60 and so the retainer may not move smoothly relative to the ball nut.
(2) An electric discharge machine and a wire cutting machine which performs a processing operation at a low speed and oscillating operation is often driven at a high-speed feeding during a period other than the processing operation in order to improve the working efficiency. In the high-speed feeding operation, the balls circulating through the dowel 60 collide with the elongated holes 42 of the cage type retainer 40 thereby to form bruises at the retainer 40, so that the torque characteristics may be deteriorated due to the influence of the bruises.
(3) When the ball moves away from the retainer 40 and then the ball slips and moves over the surface of the land portion of the screw shaft within the ball circulation path 62 to the adjacent groove, the ball traces such a locus that the ball moves once outside of a ball circle diameter (hereinafter referring to BCD) due to the centrifugal force applied thereto and again returns on the BCD. Thus, when the ball returns to the cage type retainer 40, there may arise such phenomena that the ball collides with the cage type retainer 40 with a lager force or the ball is thrust between the nut and the retainer (thrusting phenomenon like a wedge).
Accordingly, the present invention has been performed in order to solve the aforesaid unsolved problem of the conventional ball screw with a cage type retainer.
In is an object of the present invention to provide a ball screw with a cage type retainer in which the relative size of the cage type retainer to the radial direction and the hardness of the retainer are adjusted so that the ball screw can prevent the occurrence of the interference between a retainer and a dowel, collision of a ball with the retainer, thrusting of the ball or the like thereby to improve the torque efficiency.
The above-mentioned object can be attained by a ball screw according to the present invention comprising:
a screw shaft including a first ball screw groove formed in an outer peripheral surface thereof and formed into a spiral shape;
a ball nut including a second ball screw groove formed in an inner peripheral surface thereof and opposing the first ball screw groove;
a plurality of balls rollingly fitted between the first and second ball screw for allowing the ball nut to move with respect to the screw shaft;
a dowel provided at a portion of the ball screw grooves of the ball nut so as to guide a ball to move over a thread of the screw shaft and to circulate along the first and second ball screw grooves, thereby to form a circulation path; and
a cylindrical retainer separately disposed between the screw shaft and the ball nut in a radial direction of the screw shaft, and provided with a plurality of elongated holes extending in an axial direction thereof at its peripheral surface for separating the balls from one another in a circumferential direction thereof and for rollingably holding the balls therein, wherein a distance defined between the screw shaft and the retainer is smaller than a distance defined between the dowel and the retainer in the radial direction.
In the aforementioned ball screw, it is advantageous that the retainer has a hardness in a range of HRC 22-HRC 67.
Further, in the aforementioned ball screw, it is advantageous that the retainer is made of a magnetized material.